Energy feeding a fault on a DC bus or a DC grid should be controlled/limited as quickly and cost effectively as possible. All energy sources connected to the DC bus or DC grid in question have power electronic converter interfaces, and the DC voltage on the DC bus or DC grid is typically at a medium or high voltage level. DC fault current can be reduced by using a thyristor or diode based main rectifier aided by an IGBT (insulated gate bipolar transistor) based active power filter that controls the AC and/or DC side voltages. Alternatively, a DC-side cascade of a thyristor or diode based main rectifier and an IGBT based auxiliary rectifier fed from separate windings of a transformer can be used, where the auxiliary rectifier serves to control the total voltage and reduce the DC fault current. In other examples, a single converter with a main unit and a separate fault handling unit is provided.
However, thyristor and diode bridge-based solutions are only capable of unidirectional power flow. Other approaches use a single large converter made of smaller modules e.g. for high voltage applications, making converter control and module signaling more complex. A more cost-effective, simpler and robust fault-handling solution is desired for medium and high voltage applications.